The critical role of antioxidant enzymes in motor neuron survival has been established by the finding that missense mutations in the Cu/Zn superoxide dismutase (SOD) genes are responsible for a dominantly inherited form of amyotrophic lateral sclerosis (FALS). However, whether motor neuron loss in FALS results from a toxic gain in function by a mutant enzyme, or is related to diminished levels of Cu\Zn SOD activity is still unclear. The generation of transgenic mice heterozygous and homozygous for a Cu\Zn SOD gene deletion now allows definitive testing of the hypothesis that motor neuron survival in vivo is directly related to Cu\Zn SOD levels. In the work outlined, I will determine whether reductions of Cu/Zn SOD activity in these mice will lead to decreased motor neuron survival over time, preferential involvement of motor neurons as compared to other classes of neurons, and increased motor neuron vulnerability to axotomy. I will also test the idea that the motor neurons dying in these transgenic animals after axotomy will exhibit characteristics of apoptosis. The close relationship between antioxidant defenses and apoptosis is underscored by the observations that powerful inhibitors of apoptosis may in fact prevent death by antioxidant mechanisms. The bcl-2 family of genes, mammalian homologs of the ced-9 gene in C. elegans, represent one such class of molecules, and within this group there is evidence that bcl- XL may have the most relevance for the nervous system. Remarkably within another family of genes, mammalian homologs of the C. elegans ced-3 gene, one member ich-1s also appears to prevent apoptosis in vitro. Here, I propose to generate transgenic animals which overexpress either Bcl-XL or ICH-1s in motor neurons and then determine whether overexpression of these molecules is protective in acquired models of motor neuron degeneration. Furthermore, this project will determine whether overexpression of Bcl-XL or ICH-1s will preserve the phentoype of axotomized motor neurons with regard to neurotransmitter synthesis and functional reinnervation of muscle. Finally, I will compare expression of genes implicated in neuronal survival in both ALS-sensitive and ALS-resistant motor pools in an effort to identify molecules which account for the selective vulnerability of certain motor neuron populations in ALS. Overall, these experiments will test the idea that molecules related to antioxidant defenses or apoptosis regulate motor neuron survival in vivo. Results from these experiments will have important implications for the potential mechanisms underlying motor neuron disease as well as for possible therapeutic interventions.